1. The objective of this proposal is to investigate how prolactin influences the natural development of mammary tumors in mice. We find that several identifiable variants of mouse prolactin are present in the pituitary glands and sera of mice. We intend to study the significance of these variants in breast cancer development. 2. The charge isohormones of prolactin in the pituitary gland and size and immunologic variants in the serum will be characterized by electrophoresis, chromatography, radioimmunoassay, bioassay, and receptor assay methods. 3. Attempts will be made to devise methods of quantitation (radioimmunoassay) for the so far undetectable forms of prolactin in mouse blood. 4. The interrelationships between the several identifiable forms of mouse prolactin and the genetic, viral, nutritional, and chemical factors important in mouse mammary tumorigenesis will be determined.